Google Adwords is dirt cheap. You don't need a promo code. It only cost me $5 to sign up. You only pay when you want to (no monthly minimum expenditure required). You can set a $1 budget per day and 5 cents per click bids. You can disable your campaign at anytime and enable it weeks later. I think it's worth $5 just for the keywords tool.
Hello I have a $25 vochure code for adwords. We can set .10 cents per click but the refill amount is minimum 100$ to the best of my knowledge, don't know if it is changed now.
oneandone.co.uk did have a promo where if you hosted with them you got an adwords promo number but to be honest it wouldnt compensate for oneandone's rubbish customer support